A STATCOM-Control Scheme for Grid

Connected Wind Energy System for Power

Quality Improvement

Abstract

Injection of the wind power into an electric grid affects the power

quality. The performance of the wind turbine and thereby power quality

are determined on the basis of measurements and the norms followed

according to the guideline specified in International Electro-technical

Commission standard, IEC-61400. The influence of the wind turbine in

the grid system concerning the power quality measurements are-the

active power, reactive power, variation of voltage, flicker, harmonics,

and electrical behavior of switching operation and these are measured

according to national/international guidelines. The paper study

demonstrates the power quality problem due to installation of wind

turbine with the grid. In this proposed scheme STATic COMpensator

(STATCOM) is connected at a point of common coupling with a battery

energy storage system (BESS) to mitigate the power quality issues. The

battery energy storage is integrated to sustain the real power source

under fluctuating wind power. The STATCOM control scheme for the grid

connected wind energy generation system for power quality

improvement is simulated using MATLAB/SIMULINK in power system

block set. The effectiveness of the proposed scheme relives the main

supply source from the reactive power demand of the load and the

induction generator. The development of the grid co-ordination rule and

the scheme for improvement in power quality norms as per IEC-

standard on the grid has been presented.

1. INTRODUCTION

To have sustainable growth and social progress, it is necessary to meet the energy

need by utilizing the renewable energy resources like wind, biomass, hydro, co-generation,

etc. In sustainable energy system, energy conservation and the use of renewable source are

the key paradigm. The need to integrate the renewable energy like wind energy into power

system is to make it possible to minimize the environmental impact on conventional plant.

The integration of wind energy into existing power system presents a technical challenges

and that requires consideration of voltage regulation, stability, power quality problems.

The power quality is an essential customer-focused measure and is greatly affected by the

operation of a distribution and transmission network. The issue of power quality is of great

importance to the wind turbine.

There has been an extensive growth and quick development in

the exploitation of wind energy in recent years. The individual units can

be of large capacity up to 2 MW, feeding into distribution network,

particularly with customers connected in close proximity. Today, more

than 28,000 wind generating turbines are successfully operating all over

the world. In the fixed-speed wind turbine operation, all the fluctuation

in the wind speed are transmitted as fluctuations in the mechanical

torque, electrical power on the grid and leads to large voltage

fluctuations. During the normal operation, wind turbine produces a

continuous variable output power. These power variations are mainly

caused by the effect of turbulence, wind shear, and tower-shadow and

of control system in the power system.

Thus, the network needs to manage for such fluctuations. The

power quality issues can be viewed with respect to the wind generation,

transmission and distribution network, such as voltage sag, swells,

flickers, harmonics etc. However the wind generator introduces

disturbances into the distribution network. One of the simple methods of

running a wind generating system is to use the induction generator

connected directly to the grid system. The induction generator has

inherent advantages of cost effectiveness and robustness. However;

induction generators require reactive power for magnetization. When

the generated active power of an induction generator is varied due to

wind, absorbed reactive power and terminal voltage of an induction

generator can be significantly affected. A proper control scheme in wind

energy generation system is required under normal operating condition

to allow the proper control over the active power production. In the

event of increasing grid disturbance, a battery energy storage system

for wind energy generating system is generally required to compensate

the fluctuation generated by wind turbine. A STATCOM based control

technology has been proposed for improving the power quality which

can technically manages the power level associates with the

commercial wind turbines. The proposed STATCOM control scheme for

grid connected wind energy generation for power quality improvement

has following objectives.

• Unity power factor at the source side.

• Reactive power support only from STATCOM to wind Generator

and Load.

• Simple bang-bang controller for STATCOM to achieve fast

dynamic response.

The paper is organized as fallows. The Section II introduces the

power quality standards, issues and its consequences of wind turbine.

The Section III introduces the grid coordination rule for grid quality

limits. The Section IV describes the topology for power quality

improvement. The Sections V, VI, VII describes the control scheme,

system performance and conclusion respectively.

2. FACTS

Flexible AC Transmission Systems, called FACTS, got in the recent years a well

known term for higher controllability in power systems by means of power electronic

devices. Several FACTS-devices have been introduced for various applications worldwide.

A number of new types of devices are in the stage of being introduced in practice.

In most of the applications the controllability is used to avoid cost intensive or

landscape requiring extensions of power systems, for instance like upgrades or additions of

substations and power lines. FACTS-devices provide a better adaptation to varying

operational conditions and improve the usage of existing installations. The basic

applications of FACTS-devices are:

• Power flow control,

• Increase of transmission capability,

• Voltage control,

• Reactive power compensation,

• Stability improvement,

• Power quality improvement,

• Power conditioning,

• Flicker mitigation,

• Interconnection of renewable and distributed generation and storages.

Figure 1.1 shows the basic idea of FACTS for transmission systems. The usage of

lines for active power transmission should be ideally up to the thermal limits. Voltage and

stability limits shall be shifted with the means of the several different FACTS devices. It

can be seen that with growing line length, the opportunity for FACTS devices gets more

and more important.

The influence of FACTS-devices is achieved through switched or controlled shunt

compensation, series compensation or phase shift control. The devices work electrically as

fast current, voltage or impedance controllers. The power electronic allows very short

reaction times down to far below one second.

The development of FACTS-devices has started with the growing capabilities of

power electronic components. Devices for high power levels have been made available in

converters for high and even highest voltage levels. The overall starting points are network

elements influencing the reactive power or the impedance of a part of the power system.

Figure 1.2 shows a number of basic devices separated into the conventional ones and the

FACTS-devices.

For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some

explanation. The term 'dynamic' is used to express the fast controllability of FACTS-

devices provided by the power electronics. This is one of the main differentiation factors

from the conventional devices. The term 'static' means that the devices have no moving

parts like mechanical switches to perform the dynamic controllability. Therefore most of

the FACTS-devices can equally be static and dynamic.

The left column in Figure 1.2 contains the conventional devices build out of fixed

or mechanically switch able components like resistance, inductance or capacitance together

with transformers. The FACTS-devices contain these elements as well but use additional

power electronic valves or converters to switch the elements in smaller steps or with

switching patterns within a cycle of the alternating current. The left column of FACTS-

devices uses Thyristor valves or converters. These valves or converters are well known

since several years. They have low losses because of their low switching frequency of once

a cycle in the converters or the usage of the Thyristors to simply bridge impedances in the

valves.

The right column of FACTS-devices contains more advanced technology of

voltage source converters based today mainly on Insulated Gate Bipolar Transistors

(IGBT) or Insulated Gate Commutated Thyristors (IGCT). Voltage Source Converters

provide a free controllable voltage in magnitude and phase due to a pulse width modulation

of the IGBTs or IGCTs. High modulation frequencies allow to get low harmonics in the

output signal and even to compensate disturbances coming from the network. The

disadvantage is that with an increasing switching frequency, the losses are increasing as

well. Therefore special designs of the converters are required to compensate this.

Configurations of FACTS-Devices:

Shunt Devices:

The most used FACTS-device is the SVC or the version with Voltage Source

Converter called STATCOM. These shunt devices are operating as reactive power

compensators. The main applications in transmission, distribution and industrial networks

are:

• Reduction of unwanted reactive power flows and therefore reduced network

losses.

• Keeping of contractual power exchanges with balanced reactive power.

• Compensation of consumers and improvement of power quality especially with

huge demand fluctuations like industrial machines, metal melting plants, railway or

underground train systems.

• Compensation of Thyristor converters e.g. in conventional HVDC lines.

• Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs are used for

industrial applications. Industry as well as commercial and domestic groups of users

require power quality. Flickering lamps are no longer accepted, nor are interruptions of

industrial processes due to insufficient power quality. Railway or underground systems

with huge load variations require SVCs or STATCOMs.

SVC:

Electrical loads both generate and absorb reactive power. Since the transmitted load

varies considerably from one hour to another, the reactive power balance in a grid varies as

well. The result can be unacceptable voltage amplitude variations or even a voltage

depression, at the extreme a voltage collapse.

A rapidly operating Static Var Compensator (SVC) can continuously provide the

reactive power required to control dynamic voltage oscillations under various system

conditions and thereby improve the power system transmission and distribution stability.

Applications of the SVC systems in transmission systems:

a. To increase active power transfer capacity and transient stability margin

b. To damp power oscillations

c. To achieve effective voltage control

In addition, SVCs are also used

1. in transmission systems

a. To reduce temporary over voltages

b. To damp sub synchronous resonances

c. To damp power oscillations in interconnected power systems

2. in traction systems

a. To balance loads

b. To improve power factor

c. To improve voltage regulation

3. In HVDC systems

a. To provide reactive power to ac–dc converters

4. In arc furnaces

a. To reduce voltage variations and associated light flicker

Installing an SVC at one or more suitable points in the network can increase

transfer capability and reduce losses while maintaining a smooth voltage profile under

different network conditions. In addition an SVC can mitigate active power oscillations

through voltage amplitude modulation.

SVC installations consist of a number of building blocks. The most important is the

Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide

controllability. Air core reactors and high voltage AC capacitors are the reactive power

elements used together with the Thyristor valves. The step up connection of this equipment

to the transmission voltage is achieved through a power transformer.

SVC building blocks and voltage / current characteristic

In principle the SVC consists of Thyristor Switched Capacitors (TSC) and

Thyristor Switched or Controlled Reactors (TSR / TCR). The coordinated control of a

combination of these branches varies the reactive power as shown in Figure. The first

commercial SVC was installed in 1972 for an electric arc furnace. On transmission level

the first SVC was used in 1979. Since then it is widely used and the most accepted

FACTS-device.

SVC

SVC USING A TCR AND AN FC:

In this arrangement, two or more FC (fixed capacitor) banks are connected to a

TCR (thyristor controlled reactor) through a step-down transformer. The rating of the

reactor is chosen larger than the rating of the capacitor by an amount to provide the

maximum lagging vars that have to be absorbed from the system. By changing the firing

angle of the thyristor controlling the reactor from 90° to 180°, the reactive power can be

varied over the entire range from maximum lagging vars to leading vars that can be

absorbed from the system by this compensator.

SVC of the FC/TCR type:

The main disadvantage of this configuration is the significant harmonics that will

be generated because of the partial conduction of the large reactor under normal sinusoidal

steady-state operating condition when the SVC is absorbing zero MVAr. These harmonics

are filtered in the following manner. Triplex harmonics are canceled by arranging the TCR

and the secondary windings of the step-down transformer in delta connection. The

capacitor banks with the help of series reactors are tuned to filter fifth, seventh, and other

higher-order harmonics as a high-pass filter. Further losses are high due to the circulating

current between the reactor and capacitor banks.

Comparison of the loss characteristics of TSC–TCR, TCR–FC compensators and

synchronous condenser

These SVCs do not have a short-time overload capability because the reactors are

usually of the air-core type. In applications requiring overload capability, TCR must be

designed for short-time overloading, or separate thyristor-switched overload reactors must

be employed.

SVC USING A TCR AND TSC:

This compensator overcomes two major shortcomings of the earlier compensators

by reducing losses under operating conditions and better performance under large system

disturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor

bank will be 1/n times the maximum output of the SVC, thus reducing the harmonics

generated by the reactor. In those situations where harmonics have to be reduced further, a

small amount of FCs tuned as filters may be connected in parallel with the TCR.

SVC of combined TSC and TCR type

When large disturbances occur in a power system due to load rejection, there is a

possibility for large voltage transients because of oscillatory interaction between system

and the SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC

compensator. In the TSC–TCR scheme, due to the flexibility of rapid switching of

capacitor banks without appreciable disturbance to the power system, oscillations can be

avoided, and hence the transients in the system can also be avoided. The capital cost of this

SVC is higher than that of the earlier one due to the increased number of capacitor

switches and increased control complexity.

STATIC SYNCHRONOUS COMPENSATOR (STATCOM)

Introduction

The STATCOM is a solid-state-based power converter version of the SVC.

Operating as a shunt-connected SVC, its capacitive or inductive output currents can be

controlled independently from its terminal AC bus voltage. Because of the fast-switching

characteristic of power converters, STATCOM provides much faster response as compared

to the SVC. In addition, in the event of a rapid change in system voltage, the capacitor

voltage does not change instantaneously; therefore, STATCOM effectively reacts for the

desired responses. For example, if the system voltage drops for any reason, there is a

tendency for STATCOM to inject capacitive power to support the dipped voltages.

STATCOM is capable of high dynamic performance and its compensation does not

depend on the common coupling voltage. Therefore, STATCOM is very effective during

the power system disturbances.

Moreover, much research confirms several advantages of STATCOM. These

advantages compared to other shunt compensators include:

• Size, weight, and cost reduction

• Equality of lagging and leading output

• Precise and continuous reactive power control with fast response

• Possible active harmonic filter capability

This chapter describes the structure, basic operating principle and

characteristics of STATCOM. In addition, the concept of voltage source converters and the

corresponding control techniques are illustrated.

STRUCTURE OF STATCOM

Basically, STATCOM is comprised of three main parts (as seen from Figure

below): a voltage source converter (VSC), a step-up coupling transformer, and a controller.

In a very-high-voltage system, the leakage inductances of the step-up power transformers

can function as coupling reactors. The main purpose of the coupling inductors is to filter

out the current harmonic components that are generated mainly by the pulsating output

voltage of the power converters.

Reactive power generation by a STATCOM

CONTROL OF STATCOM

Introduction

The controller of a STATCOM operates the converter in a particular way that the

phase angle between the converter voltage and the transmission line voltage is dynamically

adjusted and synchronized so that the STATCOM generates or absorbs desired VAR at the

point of coupling connection. Figure 3.4 shows a simplified diagram of the STATCOM

with a converter voltage source __1E and a tie reactance, connected to a system with a

voltage source, and a Thevenin reactance, XTIEX_THVTH.

Two Modes of Operation

There are two modes of operation for a STATCOM, inductive mode and the

capacitive mode. The STATCOM regards an inductive reactance connected at its terminal

when the converter voltage is higher than the transmission line voltage. Hence, from the

system’s point of view, it regards the STATCOM as a capacitive reactance and the

STATCOM is considered to be operating in a capacitive mode. Similarly, when the system

voltage is higher than the converter voltage, the system regards an inductive reactance

connected at its terminal. Hence, the STATCOM regards the system as a capacitive

reactance and the STATCOM is considered to be operating in an inductive mode

.

STATCOM operating in inductive or capacitive modes

In other words, looking at the phasor diagrams on the right of Figure 3.4, when1I,

the reactive current component of the STATCOM, leads (THVE−1) by 90º, it is in

inductive mode and when it lags by 90º, it is in capacitive mode.

This dual mode capability enables the STATCOM to provide inductive

compensation as well as capacitive compensation to a system. Inductive compensation of

the STATCOM makes it unique. This inductive compensation is to provide inductive

reactance when overcompensation due to capacitors banks occurs. This happens during the

night, when a typical inductive load is about 20% of the full load, and the capacitor banks

along the transmission line provide with excessive capacitive reactance due to the lower

load. Basically the control system for a STATCOM consists of a current control and a

voltage control.

Current Controlled STATCOM

Current controlled block diagram of STATCOM

Figure above shows the reactive current control block diagram of the STATCOM.

An instantaneous three-phase set of line voltages, v l, at BUS 1 is used to calculate the

reference angle, θ, which is phase-locked to the phase a of the line voltage, vla . An

instantaneous three-phase set of measured converter currents, i l, is decomposed into its real

or direct component, I1d, and reactive or quadrature component, I1q, respectively. The

quadrature component is compared with the desired reference value, I1q* and the error is

passed through an error amplifier which produces a relative angle, α, of the converter

voltage with respect to the transmission line voltage. The phase angle, θ1, of the converter

voltage is calculated by adding the relative angle, α, of the converter voltage and the phase

– lock-loop angle, θ. The reference quadrature component, I1q*, of the converter current is

defined to be either positive if the STATCOM is emulating an inductive reactance or

negative if it is emulating a capacitive reactance. The DC capacitor voltage, vDC, is

dynamically adjusted in relation with the converter voltage. The control scheme described

above shows the implementation of the inner current control loop which regulates the

reactive current flow through the STATCOM regardless of the line voltage.

Voltage Controlled STATCOM

In regulating the line voltage, an outer voltage control loop must be implemented.

The outer voltage control loop would automatically determine the reference reactive

current for the inner current control loop which, in turn, will regulate the line voltage.

Voltage controlled block diagram of STATCOM

Figure shows a voltage control block diagram of the STATCOM. An instantaneous

three-phase set of measured line voltages, v1, at BUS 1 is decomposed into its real or direct

component, V1d, and reactive or quadrature component, V1q, is compared with the desired

reference value, V1*, (adjusted by the droop factor, Kdroop) and the error is passed through

an error amplifier which produces the reference current, I1q*, for the inner current control

loop. The droop factor, Kdroop, is defined as the allowable voltage error at the rated reactive

current flow through the STATCOM.

BASIC OPERATING PRINCIPLES OF STATCOM

The STATCOM is connected to the power system at a PCC (point of common

coupling), through a step-up coupling transformer, where the voltage-quality problem is a

concern. The PCC is also known as the terminal for which the terminal voltage is UT. All

required voltages and currents are measured and are fed into the controller to be compared

with the commands. The controller then performs feedback control and outputs a set of

switching signals (firing angle) to drive the main semiconductor switches of the power

converter accordingly to either increase the voltage or to decrease it accordingly. A

STATCOM is a controlled reactive-power source. It provides voltage support by

generating or absorbing reactive power at the point of common coupling without the need

of large external reactors or capacitor banks. Using the controller, the VSC and the

coupling transformer, the STATCOM operation is illustrated in Figure below.

STATCOM operation in a power system

The charged capacitor Cdc provides a DC voltage, Udc to the converter, which

produces a set of controllable three-phase output voltages, U in synchronism with the AC

system. The synchronism of the three-phase output voltage with the transmission line

voltage has to be performed by an external controller. The amount of desired voltage

across STATCOM, which is the voltage reference, Uref, is set manually to the controller.

The voltage control is thereby to match UT with Uref which has been elaborated. This

matching of voltages is done by varying the amplitude of the output voltage U, which is

done by the firing angle set by the controller. The controller thus sets UT equivalent to the

Uref. The reactive power exchange between the converter and the AC system can also be

controlled. This reactive power exchange is the reactive current injected by the

STATCOM, which is the current from the capacitor produced by absorbing real power

from the AC system.

where Iq is the reactive current injected by the STATCOM

UT is the STATCOM terminal voltage

Ueq is the equivalent Thevenin voltage seen by the STATCOM

Xeq is the equivalent Thevenin reactance of the power system seen by the

STATCOM

If the amplitude of the output voltage U is increased above that of the AC

system voltage, UT, a leading current is produced, i.e. the STATCOM is seen as a

conductor by the AC system and reactive power is generated. Decreasing the amplitude of

the output voltage below that of the AC system, a lagging current results and the

STATCOM is seen as an inductor. In this case reactive power is absorbed. If the

amplitudes are equal no power exchange takes place.

A practical converter is not lossless. In the case of the DC capacitor, the energy

stored in this capacitor would be consumed by the internal losses of the converter. By

making the output voltages of the converter lag the AC system voltages by a small angle,

δ, the converter absorbs a small amount of active power from the AC system to balance the

losses in the converter. The diagram in Figure below illustrates the phasor diagrams of the

voltage at the terminal, the converter output current and voltage in all four quadrants of the

PQ plane.

Phasor diagrams for STATCOM applications

The mechanism of phase angle adjustment, angle δ, can also be used to control

the reactive power generation or absorption by increasing or decreasing the capacitor

voltage Udc, with reference with the output voltage U.

Instead of a capacitor a battery can also be used as DC energy. In this case the

converter can control both reactive and active power exchange with the AC system. The

capability of controlling active as well as reactive power exchange is a significant feature

which can be used effectively in applications requiring power oscillation damping, to level

peak power demand, and to provide uninterrupted power for critical load.

CHARACTERISTICS OF STATCOM

The derivation of the formula for the transmitted active power employs

considerable calculations. Using the variables defined in Figure below and applying

Kirchoffs laws the following equations can be written;

Two machine system with STATCOM

By equaling right-hand terms of the above formulas, a formula for the current I1 is

obtained as

Where UR is the STATCOM terminal voltage if the STATCOM is out of operation,

i.e. when Iq = 0. The fact that Iq is shifted by 90◦ with regard to UR can be used to express Iq

as

Applying the sine law to the diagram in Figure below the following two equations

result

From which the formula for sin α is derived as

The formula for the transmitted active power can be given as

To dispose of the term UR the cosine law is applied to the diagram in Figure above

Therefore,

Transmitted power versus transmission angle characteristic of a STATCOM

With these concepts of STATCOM, it is thus important to utilize these

principles in accommodating shunt compensation to any system. Since this thesis only

reflects on the voltage control and power increase, the requirements of the STATCOM

would be further elaborated.

FUNCTIONAL REQUIREMENTS OF STATCOM

The main functional requirements of the STATCOM in this thesis are to provide

shunt compensation, operating in capacitive mode only, in terms of the following;

• Voltage stability control in a power system, as to compensate the loss voltage

along transmission. This compensation of voltage has to be in synchronism with the AC

system regardless of disturbances or change of load.

• Transient stability during disturbances in a system or a change of load.

• Direct voltage support to maintain sufficient line voltage for facilitating increased

reactive power flow under heavy loads and for preventing voltage instability

• Reactive power injection by STATCOM into the system

The design phase and implementation phase (as presented in the next chapter)

would refer to the theoretical background of STATCOM in providing the requirements

In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic

COMpensator) went into operation. The STATCOM has a characteristic similar to the

synchronous condenser, but as an electronic device it has no inertia and is superior to the

synchronous condenser in several ways, such as better dynamics, a lower investment cost

and lower operating and maintenance costs. A STATCOM is build with Thyristors with

turn-off capability like GTO or today IGCT or with more and more IGBTs. The static line

between the current limitations has a certain steepness determining the control

characteristic for the voltage.

The advantage of a STATCOM is that the reactive power provision is independent

from the actual voltage on the connection point. This can be seen in the diagram for the

maximum currents being independent of the voltage in comparison to the SVC. This

means, that even during most severe contingencies, the STATCOM keeps its full

capability.

In the distributed energy sector the usage of Voltage Source Converters for grid

interconnection is common practice today. The next step in STATCOM development is the

combination with energy storages on the DC-side. The performance for power quality and

balanced network operation can be improved much more with the combination of active

and reactive power.

STATCOM structure and voltage / current characteristic

STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize

either Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT)

devices. The STATCOM is a very fast acting, electronic equivalent of a synchronous

condenser. If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc)

is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is

smaller then Es then lagging or inductive VARS are produced.

6 Pulses STATCOM

The three phases STATCOM makes use of the fact that on a three phase,

fundamental frequency, steady state basis, and the instantaneous power entering a purely

reactive device must be zero. The reactive power in each phase is supplied by circulating

the instantaneous real power between the phases. This is achieved by firing the GTO/diode

switches in a manner that maintains the phase difference between the ac bus voltage ES

and the STATCOM generated voltage VS. Ideally it is possible to construct a device based

on circulating instantaneous power which has no energy storage device (ie no dc

capacitor).

A practical STATCOM requires some amount of energy storage to accommodate

harmonic power and ac system unbalances, when the instantaneous real power is non-zero.

The maximum energy storage required for the STATCOM is much less than for a

TCR/TSC type of SVC compensator of comparable rating.

STATCOM Equivalent Circuit

Several different control techniques can be used for the firing control of the

STATCOM. Fundamental switching of the GTO/diode once per cycle can be used. This

approach will minimize switching losses, but will generally utilize more complex

transformer topologies. As an alternative, Pulse Width Modulated (PWM) techniques,

which turn on and off the GTO or IGBT switch more than once per cycle, can be used.

This approach allows for simpler transformer topologies at the expense of higher switching

losses.

The 6 Pulse STATCOM using fundamental switching will of course produce the 6

N1 harmonics. There are a variety of methods to decrease the harmonics. These methods

include the basic 12 pulse configuration with parallel star / delta transformer connections, a

complete elimination of 5th and 7th harmonic current using series connection of star/star

and star/delta transformers and a quasi 12 pulse method with a single star-star transformer,

and two secondary windings, using control of firing angle to produce a 30phase shift

between the two 6 pulse bridges. This method can be extended to produce a 24 pulse and a

48 pulse STATCOM, thus eliminating harmonics even further. Another possible approach

for harmonic cancellation is a multi-level configuration which allows for more than one

switching element per level and therefore more than one switching in each bridge arm. The

ac voltage derived has a staircase effect, dependent on the number of levels. This staircase

voltage can be controlled to eliminate harmonics.

Substation with a STATCOM

Series Devices:

Series devices have been further developed from fixed or mechanically switched

compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage

Source Converter based devices.

The main applications are:

• Reduction of series voltage decline in magnitude and angle over a power line,

• Reduction of voltage fluctuations within defined limits during changing power

transmissions,

• Improvement of system damping resp. damping of oscillations,

• Limitation of short circuit currents in networks or substations,

• Avoidance of loop flows resp. power flow adjustments.

TCSC:

Thyristor Controlled Series Capacitors (TCSC) address specific dynamical

problems in transmission systems. Firstly it increases damping when large electrical

systems are interconnected. Secondly it can overcome the problem of Sub Synchronous

Resonance (SSR), a phenomenon that involves an interaction between large thermal

generating units and series compensated transmission systems.

The TCSC's high speed switching capability provides a mechanism for controlling

line power flow, which permits increased loading of existing transmission lines, and allows

for rapid readjustment of line power flow in response to various contingencies. The TCSC

also can regulate steady-state power flow within its rating limits.

From a principal technology point of view, the TCSC resembles the conventional

series capacitor. All the power equipment is located on an isolated steel platform, including

the Thyristor valve that is used to control the behavior of the main capacitor bank.

Likewise the control and protection is located on ground potential together with other

auxiliary systems. Figure shows the principle setup of a TCSC and its operational diagram.

The firing angle and the thermal limits of the Thyristors determine the boundaries of the

operational diagram.

Advantages

Continuous control of desired compensation level

Direct smooth control of power flow within the network

Improved capacitor bank protection

Local mitigation of sub synchronous resonance (SSR). This permits higher

levels of compensation in networks where interactions with turbine-generator torsional

vibrations or with other control or measuring systems are of concern.

Damping of electromechanical (0.5-2 Hz) power oscillations which often arise

between areas in a large interconnected power network. These oscillations are due to the

dynamics of inter area power transfer and often exhibit poor damping when the aggregate

power tranfer over a corridor is high relative to the transmission strength.

Shunt and Series Devices

Dynamic Power Flow Controller

A new device in the area of power flow control is the Dynamic Power Flow

Controller (DFC). The DFC is a hybrid device between a Phase Shifting Transformer

(PST) and switched series compensation.

A functional single line diagram of the Dynamic Flow Controller is shown in

Figure 1.19. The Dynamic Flow Controller consists of the following components:

• a standard phase shifting transformer with tap-changer (PST)

• series-connected Thyristor Switched Capacitors and Reactors

(TSC / TSR)

• A mechanically switched shunt capacitor (MSC). (This is

optional depending on the system reactive power requirements)

Based on the system requirements, a DFC might consist of a number of series TSC

or TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in

case of overload and other conditions. Normally the reactance of reactors and the

capacitors are selected based on a binary basis to result in a desired stepped reactance

variation. If a higher power flow resolution is needed, a reactance equivalent to the half of

the smallest one can be added.

The switching of series reactors occurs at zero current to avoid any harmonics.

However, in general, the principle of phase-angle control used in TCSC can be applied for

a continuous control as well. The operation of a DFC is based on the following rules:

• TSC / TSR are switched when a fast response is required.

• The relieve of overload and work in stressed situations is handled by the TSC /

TSR.

• The switching of the PST tap-changer should be minimized particularly for the

currents higher than normal loading.

• The total reactive power consumption of the device can be optimized by the

operation of the MSC, tap changer and the switched capacities and reactors.

In order to visualize the steady state operating range of the DFC, we assume an

inductance in parallel representing parallel transmission paths. The overall control

objective in steady state would be to control the distribution of power flow between the

branch with the DFC and the parallel path. This control is accomplished by control of the

injected series voltage.

The PST (assuming a quadrature booster) will inject a voltage in quadrature with

the node voltage. The controllable reactance will inject a voltage in quadrature with the

throughput current. Assuming that the power flow has a load factor close to one, the two

parts of the series voltage will be close to collinear. However, in terms of speed of control,

influence on reactive power balance and effectiveness at high/low loading the two parts of

the series voltage has quite different characteristics. The steady state control range for

loadings up to rated current is illustrated in Figure 1.20, where the x-axis corresponds to

the throughput current and the y-axis corresponds to the injected series voltage.

Fig1.20. Operational diagram of a DFC

Operation in the first and third quadrants corresponds to reduction of power

through the DFC, whereas operation in the second and fourth quadrants corresponds to

increasing the power flow through the DFC. The slope of the line passing through the

origin (at which the tap is at zero and TSC / TSR are bypassed) depends on the short

circuit reactance of the PST.

Starting at rated current (2 kA) the short circuit reactance by itself provides an

injected voltage (approximately 20 kV in this case). If more inductance is switched in

and/or the tap is increased, the series voltage increases and the current through the DFC

decreases (and the flow on parallel branches increases). The operating point moves along

lines parallel to the arrows in the figure. The slope of these arrows depends on the size of

the parallel reactance. The maximum series voltage in the first quadrant is obtained when

all inductive steps are switched in and the tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput current decreases

(due e.g. to changing loading of the system) the series voltage will decrease. At zero

current, it will not matter whether the TSC / TSR steps are in or out, they will not

contribute to the series voltage. Consequently, the series voltage at zero current

corresponds to rated PST series voltage. Next, moving into the second quadrant, the

operating range will be limited by the line corresponding to maximum tap and the

capacitive step being switched in (and the inductive steps by-passed). In this case, the

capacitive step is approximately as large as the short circuit reactance of the PST, giving an

almost constant maximum voltage in the second quadrant.

Unified Power Flow Controller:

The UPFC is a combination of a static compensator and static series compensation.

It acts as a shunt compensating and a phase shifting device simultaneously.

Fig1.21. Principle configuration of an UPFC

The UPFC consists of a shunt and a series transformer, which are connected via

two voltage source converters with a common DC-capacitor. The DC-circuit allows the

active power exchange between shunt and series transformer to control the phase shift of

the series voltage. This setup, as shown in Figure 1.21, provides the full controllability for

voltage and power flow. The series converter needs to be protected with a Thyristor bridge.

Due to the high efforts for the Voltage Source Converters and the protection, an UPFC is

getting quite expensive, which limits the practical applications where the voltage and

power flow control is required simultaneously.

OPERATING PRINCIPLE OF UPFC

The basic components of the UPFC are two voltage source inverters (VSIs) sharing

a common dc storage capacitor, and connected to the power system through coupling

transformers. One VSI is connected to in shunt to the transmission system via a shunt

transformer, while the other one is connected in series through a series transformer.

A basic UPFC functional scheme is shown in fig.1

The series inverter is controlled to inject a symmetrical three phase voltage system

(Vse), of controllable magnitude and phase angle in series with the line to control active

and reactive power flows on the transmission line. So, this inverter will exchange active

and reactive power with the line. The reactive power is electronically provided by the

series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is

operated in such a way as to demand this dc terminal power (positive or negative) from the

line keeping the voltage across the storage capacitor Vdc constant. So, the net real power

absorbed from the line by the UPFC is equal only to the losses of the inverters and their

transformers. The remaining capacity of the shunt inverter can be used to exchange

reactive power with the line so to provide a voltage regulation at the connection point.

The two VSI’s can work independently of each other by separating the dc side. So

in that case, the shunt inverter is operating as a STATCOM that generates or absorbs

reactive power to regulate the voltage magnitude at the connection point. Instead, the series

inverter is operating as SSSC that generates or absorbs reactive power to regulate the

current flow, and hence the power low on the transmission line.

The UPFC has many possible operating modes. In particular, the shunt inverter is

operating in such a way to inject a controllable current, ish into the transmission line. The

shunt inverter can be controlled in two different modes:

VAR Control Mode: The reference input is an inductive or capacitive VAR request.

The shunt inverter control translates the var reference into a corresponding shunt current

request and adjusts gating of the inverter to establish the desired current. For this mode of

control a feedback signal representing the dc bus voltage, Vdc, is also required.

Automatic Voltage Control Mode: The shunt inverter reactive current is

automatically regulated to maintain the transmission line voltage at the point of connection

to a reference value. For this mode of control, voltage feedback signals are obtained from

the sending end bus feeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the voltage injected in

series with the line to influence the power flow on the line. The actual value of the injected

voltage can be obtained in several ways.

Direct Voltage Injection Mode: The reference inputs are directly the magnitude and

phase angle of the series voltage. Phase Angle Shifter Emulation mode: The reference

input is phase displacement between the sending end voltage and the receiving end voltage.

Line Impedance Emulation mode: The reference input is an impedance value to insert in

series with the line impedance. Automatic Power Flow Control Mode: The reference inputs

are values of P and Q to maintain on the transmission line despite system changes.

MATLAB

Matlab is a high-performance language for technical computing. It integrates

computation, visualization, and programming in an easy-to-use environment where

problems and solutions are expressed in familiar mathematical notation. Typical uses

include Math and computation Algorithm development Data acquisition Modeling,

simulation, and prototyping Data analysis, exploration, and visualization Scientific and

engineering graphics Application development, including graphical user interface building.

Matlab is an interactive system whose basic data element is an array that does not

require dimensioning. This allows you to solve many technical computing problems,

especially those with matrix and vector formulations, in a fraction of the time it would take

to write a program in a scalar no interactive language such as C or Fortran.

The name matlab stands for matrix laboratory. Matlab was originally written to

provide easy access to matrix software developed by the linpack and eispack projects.

Today, matlab engines incorporate the lapack and blas libraries, embedding the state of the

art in software for matrix computation.

Matlab has evolved over a period of years with input from many users. In

university environments, it is the standard instructional tool for introductory and advanced

courses in mathematics, engineering, and science. In industry, matlab is the tool of choice

for high-productivity research, development, and analysis.

Matlab features a family of add-on application-specific solutions called toolboxes.

Very important to most users of matlab, toolboxes allow you to learn and apply specialized

technology. Toolboxes are comprehensive collections of matlab functions (M-files) that

extend the matlab environment to solve particular classes of problems. Areas in which

toolboxes are available include signal processing, control systems, neural networks, fuzzy

logic, wavelets, simulation, and many others.

The matlab system consists of five main parts:

Development Environment. This is the set of tools and facilities that help you use

matlab functions and files. Many of these tools are graphical user interfaces. It includes the

matlab desktop and Command Window, a command history, an editor and debugger, and

browsers for viewing help, the workspace, files, and the search path.

The matlab Mathematical Function Library. This is a vast collection of

computational algorithms ranging from elementary functions, like sum, sine, cosine, and

complex arithmetic, to more sophisticated functions like matrix inverse, matrix

eigenvalues, Bessel functions, and fast Fourier transforms.

The matlab Language. This is a high-level matrix/array language with control

flow statements, functions, data structures, input/output, and object-oriented programming

features. It allows both "programming in the small" to rapidly create quick and dirty throw-

away programs, and "programming in the large" to create large and complex application

programs.

Matlab has extensive facilities for displaying vectors and matrices as graphs, as

well as annotating and printing these graphs. It includes high-level functions for two-

dimensional and three-dimensional data visualization, image processing, animation, and

presentation graphics. It also includes low-level functions that allow you to fully customize

the appearance of graphics as well as to build complete graphical user interfaces on your

matlab applications.

The matlab Application Program Interface (API). This is a library that allows you

to write C and Fortran programs that interact with matlab. It includes facilities for calling

routines from matlab (dynamic linking), calling matlab as a computational engine, and for

reading and writing MAT-files.

4.2 SIMULINK:

4.2.1 Introduction:

Simulink is a software add-on to matlab which is a mathematical tool developed by

The Math works,(http://www.mathworks.com) a company based in Natick. Matlab is

powered by extensive numerical analysis capability. Simulink is a tool used to visually

program a dynamic system (those governed by Differential equations) and look at results.

Any logic circuit, or control system for a dynamic system can be built by using standard

building blocks available in Simulink Libraries. Various toolboxes for different techniques,

such as Fuzzy Logic, Neural Networks, dsp, Statistics etc. are available with Simulink,

which enhance the processing power of the tool. The main advantage is the availability of

templates / building blocks, which avoid the necessity of typing code for small

mathematical processes.

Concept of signal and logic flow:

In Simulink, data/information from various blocks are sent to another block by

lines connecting the relevant blocks. Signals can be generated and fed into blocks

dynamic / static).Data can be fed into functions. Data can then be dumped into sinks,

which could be scopes, displays or could be saved to a file. Data can be connected from

one block to another, can be branched, multiplexed etc. In simulation, data is processed

and transferred only at Discrete times, since all computers are discrete systems. Thus, a

simulation time step (otherwise called an integration time step) is essential, and the

selection of that step is determined by the fastest dynamics in the simulated system.

Fig 4.1 Simulink library browser

Connecting blocks:

fig 4.2 Connectung blocks

To connect blocks, left-click and drag the mouse from the output of one block to

the input of another block.

Sources and sinks:

The sources library contains the sources of data/signals that one would use in a

dynamic system simulation. One may want to use a constant input, a sinusoidal wave, a

step, a repeating sequence such as a pulse train, a ramp etc. One may want to test

disturbance effects, and can use the random signal generator to simulate noise. The clock

may be used to create a time index for plotting purposes. The ground could be used to

connect to any unused port, to avoid warning messages indicating unconnected ports.

The sinks are blocks where signals are terminated or ultimately used. In most cases,

we would want to store the resulting data in a file, or a matrix of variables. The data could

be displayed or even stored to a file. the stop block could be used to stop the simulation if

the input to that block (the signal being sunk) is non-zero. Figure 3 shows the available

blocks in the sources and sinks libraries. Unused signals must be terminated, to prevent

warnings about unconnected signals.

fig 4.3 Sources and sinks

Continuous and discrete systems:

All dynamic systems can be analyzed as continuous or discrete time systems.

Simulink allows you to represent these systems using transfer functions, integration blocks,

delay blocks etc.

fig 4.4 continous and descrete systems

Non-linear operators:

A main advantage of using tools such as Simulink is the ability to simulate non-

linear systems and arrive at results without having to solve analytically. It is very difficult

to arrive at an analytical solution for a system having non-linearities such as saturation,

signup function, limited slew rates etc. In Simulation, since systems are analyzed using

iterations, non-linearities are not a hindrance. One such could be a saturation block, to

indicate a physical limitation on a parameter, such as a voltage signal to a motor etc.

Manual switches are useful when trying simulations with different cases. Switches are the

logical equivalent of if-then statements in programming.

fig 4.5 simulink blocks

Mathematical operations:

Mathematical operators such as products, sum, logical operations such as and, or,

etc. .can be programmed along with the signal flow. Matrix multiplication becomes easy

with the matrix gain block. Trigonometric functions such as sin or tan inverse (at an) are

also available. Relational operators such as ‘equal to’, ‘greater than’ etc. can also be used

in logic circuits

fig 4.6 Simulink math blocks

4.6.2 SIGNALS & DATA TRANSFER:

In complicated block diagrams, there may arise the need to transfer data from one

portion to another portion of the block. They may be in different subsystems. That signal

could be dumped into a goto block, which is used to send signals from one subsystem to

another.

Multiplexing helps us remove clutter due to excessive connectors, and makes

matrix(column/row) visualization easier.

fig 4.7 signals and systems

4.6.3 Making subsystems

Drag a subsystem from the Simulink Library Browser and place it in the parent

block where you would like to hide the code. The type of subsystem depends on the

purpose of the block. In general one will use the standard subsystem but other subsystems

can be chosen. For instance, the subsystem can be a triggered block, which is enabled only

when a trigger signal is received.

Open (double click) the subsystem and create input / output PORTS, which transfer

signals into and out of the subsystem. The input and output ports are created by dragging

them from the Sources and Sinks directories respectively. When ports are created in the

subsystem, they automatically create ports on the external (parent) block. This allows for

connecting the appropriate signals from the parent block to the subsystem.

4.6.4 Setting simulation parameters:

Running a simulation in the computer always requires a numerical technique to

solve a differential equation. The system can be simulated as a continuous system or a

discrete system based on the blocks inside. The simulation start and stop time can be

specified. In case of variable step size, the smallest and largest step size can be specified. A

Fixed step size is recommended and it allows for indexing time to a precise number of

points, thus controlling the size of the data vector. Simulation step size must be decided

based on the dynamics of the system. A thermal process may warrant a step size of a few

seconds, but a DC motor in the system may be quite fast and may require a step size of a

few milliseconds.

3. POWER QUALITY STANDARDS, ISSUESAND IT’S CONSEQUENCES

A. International Electro Technical Commission Guidelines

The guidelines are provided for measurement of power quality of wind turbine. The

International standards are developed by the working group of Technical Committee-88 of

the International Electro-technical Commission (IEC), IEC standard 61400-21, describes

the procedure for determining the power quality characteristics of the wind turbine . The

standard norms are specified.

1) IEC 61400-21: Wind turbine generating system, part-21. Measurement and

Assessment of power quality characteristic of grid connected wind turbine

2) IEC 61400-13: Wind Turbine measuring procedure in determining the power

behavior.

3) IEC 61400-3-7: Assessment of emission limits for fluctuating load IEC 61400-

12: Wind Turbine performance. The data sheet with electrical characteristic of wind

turbine provides the base for the utility assessment regarding a grid connection.

B. Voltage Variation

The voltage variation issue results from the wind velocity and generator torque.

The voltage variation is directly related to real and reactive power variations. The voltage

variation is commonly classified as under:

• Voltage Sag/Voltage Dips.

• Voltage Swells.

• Short Interruptions.

• Long duration voltage variation.

The voltage flicker issue describes dynamic variations in the network caused by

wind turbine or by varying loads. Thus the power fluctuation from wind turbine occurs

during continuous operation. The amplitude of voltage fluctuation depends on grid

strength, network impedance, and phase-angle and power factor of the wind turbines. It is

defined as a fluctuation of voltage in a frequency 10–35 Hz. The IEC 61400-4-15 specifies

a flicker

C. Harmonics

The harmonic results due to the operation of power electronic converters. The

harmonic voltage and current should be limited to the acceptable level at the point of wind

turbine connection to the network. To ensure the harmonic voltage within limit, each

source of harmonic current can allow only a limited contribution, as per the IEC-61400-36

guideline. The rapid switching gives a large reduction in lower order harmonic current

compared to the line commutated converter, but the output current will have high

frequency current and can be easily filter-out.

D. Wind Turbine Location in Power System

The way of connecting the wind generating system into the power system highly

influences the power quality. Thus the operation and its influence on power system depend

on the structure of the adjoining power network.

E. Self Excitation of Wind Turbine Generating System

The self excitation of wind turbine generating system (WTGS) with an

asynchronous generator takes place after disconnection of wind turbine generating system

(WTGS) with local load. The risk of self excitation arises especially when WTGS is

equipped with compensating capacitor. The capacitor connected to induction generator

provides reactive power compensation. However the voltage and frequency are determined

by the balancing of the system. The disadvantages of self excitation are the safety aspect

and balance between real and reactive power.

F. Consequences of the Issues

The voltage variation, flicker, harmonics causes the malfunction of equipments

namely microprocessor based control system, programmable logic controller; adjustable

speed drives, flickering of light and screen. It may leads to tripping of contractors, tripping

of protection devices, stoppage of sensitive equipments like personal computer,

programmable logic control system and may stop the process and even can damage of

sensitive equipments. Thus it degrades the power quality in the grid.

III. GRID COORDINATION RULE

The American Wind Energy Association (AWEA) led the effort in the united state

for adoption of the grid code for the interconnection of the wind plants to the utility

system. The first grid code was focused on the distribution level, after the blackout in the

United State in August 2003. The United State wind energy industry took a stand in

developing its own grid code for contributing to a stable grid operation. The rules for

realization of grid operation of wind generating system at the distribution network are

defined as-per IEC-61400-21. The grid quality characteristics and limits are given for

references that the customer and the utility grid may expect. According to Energy-

Economic Law, the operator of transmission grid is responsible for the organization and

operation of interconnected system.

1) Voltage Rise (u):

The voltage rise at the point of common coupling can be approximated as a function of

maximum apparent power of the turbine, the grid impedances R and X at the point of

common coupling and the phase angle [7], given in (1)

Where Φ-phase difference, U-is

the nominal voltage of grid. The Limiting voltage rise value is <2%

2) Voltage Dips (d):

The voltage dips is due to start up of wind turbine and it causes a sudden reduction of

voltage. It is the relative %voltage change due to switching operation of wind turbine. The

decrease of nominal voltage change is given in (2).

Where d is relative voltage change, rated apparent power, short circuit apparent power,

and sudden voltage reduction factor. The acceptable voltage dips limiting value is ≤3%.

3) Flicker:

The measurements are made for maximum number of specified switching operation

of wind turbine with 10-min period and 2-h period are specified, as given in (3)

Where Long term flicker. Flicker coefficient calculated from Rayleigh

distribution of the wind speed. The Limiting Value for flicker coefficient is about , for

average time of 2 h .

4) Harmonics:

The harmonic distortion is assessed for variable speed turbine with a electronic

power converter at the point of common connection [9]. The total harmonic voltage

distortion of voltage is given as in (4):

Where Vn is the nth harmonic voltage and V1 is the fundamental frequency (50) Hz. The

THD limit for 132 KV is <3 %.

THD of current is given as in (5)

Where In is the nth harmonic current and I1 is the fundamental frequency (50) Hz. The

THD of current and limit for 132 KV is <2.5%.

5) Grid Frequency:

The grid frequency in India is specified in the range of 47.5–51.5 Hz, for wind farm

connection. The wind farm shall able to withstand change in frequency up to 0.5 Hz/s.

IV. TOPOLOGY FOR POWER QUALITY

IMPROVEMENT

The STATCOM based current control voltage source inverter

injects the current into the grid in such a way that the source current

are harmonic free and their phase-angle with respect to source voltage

has a desired value. The injected current will cancel out the reactive

part and harmonic part of the load and induction generator current, thus

it improves the power factor and the power quality. To accomplish these

goals, the grid voltages are sensed and are synchronized in generating

the current command for the inverter. The proposed grid connected

system is implemented for power quality improvement at point of

common coupling (PCC), as shown in Fig. 1. The grid connected system

in Fig. 1, consists of wind energy generation system and battery energy

storage system with STATCOM.

A. Wind Energy Generating System

In this configuration, wind generations are based on constant speed topologies with

pitch control turbine. The induction generator is used in the proposed scheme because of

its simplicity, it does not require a separate field circuit, it can accept constant and variable

loads, and has natural protection against short circuit. The available power of wind energy

system is presented as under in (6).

Where p (kg/m ) is the air density and A (m2 ) is the area swept out by turbine

blade, Vwind is the wind speed in mtr/s. It is not possible to extract all kinetic energy of

wind, thus it extract a fraction of power in wind, called power coefficient Cp of the wind

turbine, and is given in (7).

Where Cp is the power coefficient, depends on type and operating condition of

wind turbine. This coefficient can be express as a function of tip speed ratio and pitch

angle. The mechanical power produce by wind turbine is given in (8)

Where R is the radius of the blade (m).

B. BESS-STATCOM

The battery energy storage system (BESS) is used as an energy storage element for

the purpose of voltage regulation. The BESS will naturally maintain dc capacitor voltage

constant and is best suited in STATCOM since it rapidly injects or absorbed reactive

power to stabilize the grid ystem. It also controls the distribution and transmission system

in a very fast rate. When power fluctuation occurs in the system, the BESS can be used to

level the power fluctuation by charging and discharging operation. The battery is

connected in parallel to the dc capacitor of STATCOM [10]–[14]. The STATCOM is a

three-phase voltage source inverter having the capacitance on its DC link and connected at

the point of common coupling. The STATCOM injects a compensating current of variable

magnitude and frequency component at the bus of common coupling.

C. System Operation

The shunt connected STATCOM with battery energy storage is connected with the

interface of the induction generator and non-linear load at the PCC in the grid system. The

STATCOM compensator output is varied according to the controlled strategy, so as to

maintain the power quality norms in the grid system. The current control strategy is

included in the control scheme that defines the functional operation of the STATCOM

compensator in the power system. A single STATCOM using insulated gate bipolar

transistor is proposed to have a reactive power support, to the induction generator and to

the nonlinear load in the grid system. The main block diagram of the system operational

scheme is shown in Fig. 2.

V. CONTROL SCHEME

The control scheme approach is based on injecting the currents

into the grid using “bang-bang controller.” The controller uses a

hysteresis current controlled technique. Using such technique, the

controller keeps the control system variable between boundaries of

hysteresis area and gives correct switching signals for STATCOM

operation.

The control system scheme for generating the switching signals to

the STATCOM is shown in Fig. 3.

The control algorithm needs the measurements of several

variables such as three-phase source current isabc, DC voltage Vdc ,

inverter current isabc with the help of sensor. The current control block,

receives an input of reference current isabc* and actual current isabc are

subtracted so as to activate the operation of STATCOM in current control

mode [16]–[18].

A. Grid Synchronization

In three-phase balance system, the RMS voltage source amplitude is calculated

at the sampling frequency from the source phase voltage ( Vsa Vsb Vsc ) and is

expressed, as sample template Vsm, sampled peak voltage, as in (9).

The in-phase unit vectors are obtained from AC source—phase voltage and the

RMS value of unit vector as shown in (10).

The in-phase generated reference currents are derived using in-phase unit voltage

template as, in (11)

Where I is proportional to magnitude of filtered source voltage for respective

phases. This ensures that the source current is controlled to be sinusoidal. The unit

vectors implement the important function in the grid connection for the

synchronization for STATCOM. This method is simple, robust and favorable as

compared with other methods [18].

B. Bang-Bang Current Controller

Bang-Bang current controller is implemented in the current control scheme.

The reference current is generated as in (10) and actual current are detected by

current sensors and are subtracted for obtaining a current error for a hysteresis based

bang-bang controller. Thus the ON/OFF switching signals for IGBT of STATCOM are

derived from hysteresis controller [19].

The switching function Sa for phase ‘a’ is expressed as (12).

Where HB is a hysteresis current-band, similarly the switching function can be

derived for phases “b” and “c”.

VI. SYSTEM PERFORMANCE

The proposed control scheme is simulated using SIMULINK in power system

block set. The system parameter for given system is given Table I. The system

performance of proposed system under dynamic condition is also presented.

A. Voltage Source Current Control—Inverter Operation

The three phase injected current into the grid from STATCOM will

cancel out the distortion caused by the nonlinear load and wind

generator. The IGBT based three-phase inverter is connected to grid

through the transformer. The generation of switching signals from

reference current is simulated within hysteresis band of 0.08. The

choice of narrow hysteresis band switching in the system improves the

current quality. The control signal of switching frequency within its

operating band, as shown in Fig. 4.

The choice of the current band depends on the operating voltage

and the interfacing transformer impedance. The compensated current

for the nonlinear load and demanded reactive power is provided by the

inverter. The real power transfer from the batteries is also supported by

the controller of this inverter. The three phase inverter injected current

are shown in Fig. 5.

B. STATCOM—Performance Under Load Variations

The wind energy generating system is connected with grid having the nonlinear

load. The performance of the system is measured by switching the STATCOM at time

t=0.7s in the system and how the STATCOM responds to the step change command for

increase in additional load at 1.0 s is shown in the simulation. When STATCOM

controller is made ON, without change in any other load condition parameters, it starts

to mitigate for reactive demand as well as harmonic current. The dynamic

performance is also carried out by step change in a load, when applied at 1.0 s. This

additional demand is fulfill by STATCOM compensator. Thus, STATCOM can regulate

the available real power from source. The result of source current, load current are

shown in Fig. 6(a) and (b) respectively. While the result of injected current from

STATCOM is shown in Fig. 6(c) and the generated current from wind generator at PCC

are depicted in Fig. 6(d).

The DC link voltage regulates the source current in the grid system, so the DC

link voltage is maintained constant across the capacitor as shown in Fig. 7(a). The

current through the dc link capacitor indicating the charging and discharging operation

as shown in Fig. 7(b)

C. Power Quality Improvement

It is observed that the source current on the grid is affected due to the effects of

nonlinear load and wind generator, thus purity of waveform may be lost on both sides in

the system. The inverter output voltage under STATCOM operation with load variation is

shown in Fig. 8. The dynamic load does affect the inverter output voltage. The source

current with and without STATCOM operation is shown in Fig. 9.

This shows that the unity power factor is maintained for the source power when the

STATCOM is in operation. The current waveform before and after the STATCOM

operation is analyzed. The Fourier analysis of this waveform is expressed and the THD of

this source current at PCC without STATCOM is 4.71%, as shown in Fig. 10.

The power quality improvement is observed at point of common coupling, when

the controller is in ON condition. The STATCOM is placed in the operation at 0.7 s and

source current waveform is shown in Fig. 11 with its FFT. It is shown that the THD has

been improved considerably and within the norms of the standard.

The above tests with proposed scheme has not only power quality improvement

feature but it also has sustain capability to support the load with the energy storage through

the batteries.

VII. CONCLUSION

The paper presents the STATCOM-based control scheme for power

quality improvement in grid connected wind generating system and with

non linear load. The power quality issues and its consequences on the

consumer and electric utility are presented. The operation of the control

system developed for the STATCOM-BESS in MATLAB/SIMULINK for

maintaining the power quality is simulated. It has a capability to cancel

out the harmonic parts of the load current. It maintains the source

voltage and current in-phase and support the reactive power demand

for the wind generator and load at PCC in the grid system, thus it gives

an opportunity to enhance the utilization factor of transmission line. The

integrated wind generation and STATCOM with BESS have shown the

outstanding performance. Thus the proposed scheme in the grid

connected system fulfills the power quality norms as per the IEC

standard 61400-21.